mcpg: default to MIS+Roughness and p_guiding 0.5; tests: request the features the material shaders need

Measured on the Bitterli pbrt scenes (1280x720, 3 reps, 8192-iteration
reference-mode references with the reference's own MC noise subtracted):
MIS+Roughness is never worse than MIS and is much better where bare glossy
conductors exist (kitchen, alpha 0.05-0.1: relMSE -27 % at 1024 iterations with
p=0.5, -55 % with p=0.9); it is a tie on living-room, whose glossy surfaces are
either gated off or diffuse-dominated composites, and identical on the diffuse
cornell-box.

p_guiding drops 0.85 -> 0.5 (and examples/renderers/mcpg.json 0.9 -> 0.5): at
p=0.9 guiding is a large net loss on both glossy scenes, +584 %/+750 % relMSE
against no guiding at 4096 iterations. Quake pins 0.9 in its own quake.json
(merian-plugin-quake feat/mcpg-guiding-prob).

test-material-system and test-slang-hot-reload both aborted on a validation
error: the material shaders declare 16-bit StorageBuffer variables and use
non-uniform sampled-image indexing, but these bare test contexts requested no
features (test_pbrt_scene/test_gltf_scene already do this). Pre-existing on
main, unrelated to the guiding change.

Author: LDAP

examples/renderers/mcpg.json

@@ -23,7 +23,7 @@
                 "6": true,
                 "7": true,
                 "emission on primary": true,
-                "guiding prob": 0.9,
+                "guiding prob": 0.5,
                 "lc": {
                     "LC buffer size": 8000009,
                     "LC probe count": 2,

include/merian-graph/nodes/render_pt_mcpg/render_pt_mcpg.hpp

@@ -132,7 +132,7 @@ class RenderMCPG : public Node {
     // --- Guiding Markov chain ---
     float dir_guide_prior = 0.2f;
     int32_t mc_samples = 5;
-    float p_guiding = 0.85f; // probability to sample the guiding distribution instead of the BSDF
+    float p_guiding = 0.5f; // probability to sample the guiding distribution instead of the BSDF
     bool missing_light_heuristic = true;
 
     uint32_t mc_adaptive_buffer_size = 32777259;
@@ -141,7 +141,7 @@ class RenderMCPG : public Node {
     uint32_t mc_locality_bits = 3;
 
     GuidingDirectionalSamplingType guiding_directional_sampling_type =
-        GuidingDirectionalSamplingType::MIS;
+        GuidingDirectionalSamplingType::MIS_ROUGHNESS;
     // Guiding is skipped below this GGX alpha, where the vMF mixture cannot represent the lobe.
     // Compared against pow2(BSDF::roughness), so 0.05 matches Cycles' default (alpha units too).
     float guiding_roughness_threshold = 0.05f;

tests/test_material_system.cpp

@@ -33,6 +33,14 @@ class MaterialSystemTest : public ::testing::Test {
     static void SetUpTestSuite() {
         spdlog::set_level(spdlog::level::debug);
         ContextCreateInfo info{
+            // Materials store fp16 (half3 tint/emission), so the shaders declare 16-bit
+            // StorageBuffer variables; without these the validation layer rejects them.
+            .features =
+                VulkanFeatures({"scalarBlockLayout", "shaderInt64", "shaderInt16", "shaderFloat16",
+                                "storageBuffer16BitAccess", "uniformAndStorageBuffer16BitAccess",
+                                "storageBuffer8BitAccess", "uniformAndStorageBuffer8BitAccess",
+                                "shaderSampledImageArrayNonUniformIndexing",
+                                "runtimeDescriptorArray"}),
             .context_extensions = {ExtensionVkValidationLayers::name, ExtensionResources::name},
             .application_name = "test-material-system",
         };

tests/test_slang_hot_reload.cpp

@@ -33,6 +33,14 @@ class SlangHotReloadTest : public ::testing::Test {
     static void SetUpTestSuite() {
         spdlog::set_level(spdlog::level::debug);
         ContextCreateInfo info{
+            // Materials store fp16 (half3 tint/emission), so the shaders declare 16-bit
+            // StorageBuffer variables; without these the validation layer rejects them.
+            .features =
+                VulkanFeatures({"scalarBlockLayout", "shaderInt64", "shaderInt16", "shaderFloat16",
+                                "storageBuffer16BitAccess", "uniformAndStorageBuffer16BitAccess",
+                                "storageBuffer8BitAccess", "uniformAndStorageBuffer8BitAccess",
+                                "shaderSampledImageArrayNonUniformIndexing",
+                                "runtimeDescriptorArray"}),
             .context_extensions = {ExtensionVkValidationLayers::name, ExtensionResources::name},
             .application_name = "test-slang-hot-reload",
         };